Supported catalyst, carbon nanotube assembly, and preparation method therefor

ABSTRACT

The present invention relates to an impregnated supported catalyst, a carbon nanotube aggregate, and a method for producing the carbon nanotube aggregate. The carbon nanotube aggregate includes a four-component catalyst in which catalytic components and active components are supported on a granular support, and bundle type carbon nanotubes grown on the catalyst. The carbon nanotube aggregate has an average particle diameter of 100 to 800 μm, a bulk density of 80 to 250 kg/m 3 , and a spherical or potato-like shape.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a supported catalyst, a carbon nanotubeaggregate, and a method for producing the carbon nanotube aggregate.

2. Description of the Related Art

Carbon nanostructures (CNSs) refer collectively to nano-sized carbonstructures having various shapes, such as nanotubes, nanohairs,fullerenes, nanocones, nanohorns, and nanorods. Carbon nanostructurescan be widely utilized in a variety of technological applicationsbecause they possess excellent characteristics.

Carbon nanotubes (CNTs) are tubular materials consisting of carbon atomsarranged in a hexagonal pattern and have a diameter of approximately 1to 100 nm. Carbon nanotubes exhibit insulating, conducting orsemiconducting properties depending on their inherent chirality. Carbonnanotubes have a structure in which carbon atoms are strongly covalentlybonded to each other. Due to this structure, carbon nanotubes have atensile strength approximately 100 times that of steel, are highlyflexible and elastic, and are chemically stable.

Carbon nanotubes are divided into three types: single-walled carbonnanotubes (SWCNTs) consisting of a single sheet and having a diameter ofabout 1 nm; double-walled carbon nanotubes (DWCNTs) consisting of twosheets and having a diameter of about 1.4 to about 3 nm; andmulti-walled carbon nanotubes (MWCNTs) consisting of three or moresheets and having a diameter of about 5 to about 100 nm.

Carbon nanotubes are being investigated for their commercialization andapplication in various industrial fields, for example, aerospace, fuelcell, composite material, biotechnology, pharmaceutical,electrical/electronic, and semiconductor industries, due to their highchemical stability, flexibility and elasticity. However, carbonnanotubes have a limitation in directly controlling the diameter andlength to industrially applicable dimensions for practical use owing totheir primary structure. Accordingly, the industrial application and useof carbon nanotubes are limited despite their excellent physicalproperties.

Carbon nanotubes are generally produced by various techniques, such asarc discharge, laser ablation, and chemical vapor deposition. However,arc discharge and laser ablation are not appropriate for mass productionof carbon nanotubes and require high arc production costs or expensivelaser equipment. Chemical vapor deposition using a vapor dispersioncatalyst has the problems of a very low synthesis rate and too small asize of final CNT particles. Chemical vapor deposition using asubstrate-supported catalyst suffers from very low efficiency in theutilization of a reactor space, thus being inappropriate for massproduction of CNTs. Thus, studies on catalysts and reaction conditionsfor chemical vapor deposition are currently underway to increase theyield of carbon nanotubes.

Catalytically active components of the catalysts usually take the formof oxides, partially or completely reduced products, or hydroxides. Thecatalysts may be, for example, supported catalysts or coprecipitatedcatalysts, which can be commonly used for CNT production. Supportedcatalysts are preferably used for the following reasons: supportedcatalysts have a higher inherent bulk density than coprecipitatedcatalysts; unlike coprecipitated catalysts, supported catalysts producea small amount of a fine powder with a size of 10 microns or less, whichreduces the possibility of occurrence of a fine powder due to attritionduring fluidization; and high mechanical strength of supported catalystseffectively stabilizes the operation of reactors.

Many methods have been proposed to prepare supported catalysts. Forexample, a supported catalyst is prepared by an impregnation methodincluding mixing an aqueous metal solution with a support, followed bycoating-drying. This method has the disadvantage that the amount of thecatalyst loaded is limited. Non-uniform distribution of activecomponents and catalytic components greatly affects CNT growth yield andCNT diameter distribution, but no technique has been proposed to controlthe non-uniform distribution.

Particularly, a supported catalyst prepared by the conventionalimpregnation method can be used for the synthesis of carbon nanotubes.In this case, however, the yield of carbon nanotubes is less than 1000%and the amount of the catalyst loaded is large, showing a limitedinfluence on yield. The catalyst is a bundle type with low bulk densityand the feeding rate of reaction gas is thus lowered, resulting in lowCNT productivity.

Under these circumstances, more research needs to be done to synthesizecarbon nanotubes with high bulk density in high yield despite the use ofsupported catalysts.

SUMMARY OF THE INVENTION

The present invention has been made in an effort to overcome thedisadvantage of low CNT yield encountered in the use of conventionalsupported catalysts, and it is an object of the present invention toprovide carbon nanotubes whose bulk density and yield are improved bysimultaneously controlling the activity of a catalyst and the amount ofa fine powder, and a method for producing the carbon nanotubes.

One aspect of the present invention provides an impregnated supportedcatalyst prepared by sequentially adding a multi-carboxylic acid andprecursors of first and second catalytic components to precursors offirst and second active components to obtain a transparent aqueous metalsolution, impregnating an aluminum-based granular support with thetransparent aqueous metal solution, followed by drying and calcination,wherein the supported catalyst has a bulk density of 0.8 to 1.5 g/cm³.

Another aspect of the present invention provides a carbon nanotubeaggregate including a four-component catalyst in which catalyticcomponents and active components are supported on a granular support,and bundle type carbon nanotubes grown on the catalyst wherein thecarbon nanotube aggregate has an average particle diameter of 100 to 800μm, a bulk density of 80 to 250 kg/m³, and a spherical or potato-likeshape.

The carbon nanotubes may have an aspect ratio of 0.9 to 1 and a stranddiameter of 10 to 50 nm.

The catalyst may have an average aspect ratio (A_(CAT)) of 1.2 or lessand the carbon nanotube aggregate may have an average aspect ratio(A_(CNT)) of 1.2 or less.

The bundle type carbon nanotubes grown on the aluminum-based granularsupport may have a particle size distribution (Dcnt) of 0.5 to 1.0.

The four-component catalyst includes first and second catalyticcomponents and first and second active components, and the number ofmoles (x) of the first catalytic component, the number of moles (y) ofthe second catalytic component, the number of moles (p) of the firstactive component, and the number of moles (q) of the second activecomponent with respect to 100 moles of the support may satisfy thefollowing relationships:

10≦x≦40;

1≦y≦20;

0.1≦y/[x+y]≦0.5;

1≦p+q≦20; and

0.1≦[p+q]/[x+y]≦0.5.

The granular support may have a bulk density of 0.6 to 1.2 g/cm³, andthe catalyst in which the catalytic components and the active componentsare supported may have a bulk density of 0.8 to 1.5 g/cm³.

The granular support may have an aspect ratio of 1.2 or less, and theaverage aspect ratio (As) of the support before the catalytic componentsand the active components are supported on the support and the averageaspect ratio (A_(CAT)) of the catalyst after the catalytic componentsand the active components are supported on the support may satisfy0.8≦A_(CAT)/As≦1.2.

The multi-carboxylic acid may be used in an amount of 0.2 to 2.0 moles,assuming that the sum of the moles (p+q) of the first and second activecomponents equals to 1.

The multi-carboxylic acid may be selected from dicarboxylic acids,tricarboxylic acids, tetracarboxylic acids, and mixtures thereof.

The calcination may be performed at 650 to 800° C.

The transparent aqueous metal solution may have a concentration of 0.01to 0.4 g/ml.

According to one embodiment, the first catalytic component may be cobalt(Co), the second catalytic component may be selected from iron (Fe),nickel (Ni), and a mixture thereof, the first active component may bemolybdenum (Mo), and the second active component may be vanadium (V).

The first and second active components may be in a weight ratio of6-0.1:0.1-6.

According to one embodiment, the catalyst may have a structure in whichthe surface and pores of the aluminum-based support are coated with amonolayer or multilayer of the catalytic components and the activecomponents, and the amount of a fine powder having a number averageparticle diameter not larger than 32 μm, as measured after ultrasonicshaking at 40 watts for 1 minute, may be 10% or less of the amount ofthe catalyst.

Yet another aspect of the present invention provides a method forproducing a carbon nanotube aggregate, including 1) sequentiallyblending a multi-carboxylic acid component and an aqueous solution ofprecursors of first and second catalytic components with an aqueoussolution of precursors of first and second active components to obtain atransparent aqueous metal solution, and mixing an aluminum-basedgranular support with the transparent aqueous metal solution, 2) dryingthe mixture under vacuum at 40 to 80° C. and calcining the dried mixtureat 650 to 800° C. to obtain a catalyst for carbon nanotube production inwhich the surface and pores of the aluminum-based support areimpregnated and coated with the catalytic components and the activecomponents, 3) feeding the catalyst for carbon nanotube production intoa fluidized bed reactor and introducing at least one carbon sourceselected from C₁-C₄ saturated or unsaturated hydrocarbons, andoptionally together with a mixed gas hydrogen and nitrogen, into thereactor at 500 to 900° C., and 4) decomposing the carbon source andgrowing carbon nanotubes on the catalyst surface by chemical vaporsynthesis.

According to one embodiment, the method may further include aging at 45to 80° C. before the drying under vacuum in step 2).

The method may further include preliminarily calcining at 250 to 400° C.before the calcination in step 2).

The method may further include impregnating a portion of the totalamount of the aqueous metal solution into the aluminum-based granularsupport just before the preliminary calcination and impregnating theremainder of the aqueous metal solution into the aluminum-based granularsupport just before the calcination.

Details of other embodiments of the present invention are included inthe detailed description that follows.

The present invention can overcome the disadvantage of low CNT yieldencountered in the use of conventional impregnated catalysts for carbonnanotube production. The supported catalyst of the present invention hascontrolled activity and can produce a controlled amount of a finepowder, enabling the synthesis of bundle type carbon nanotubes in highyield. Therefore, the supported catalyst of the present invention canfind application in various fields, such as energy materials, functionalcomposites, pharmaceuticals, batteries, and semiconductors, where carbonnanotubes are used.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a SEM image showing a bulk shape of CNTs produced in Example1-1;

FIGS. 2 a and 2 b are SEM images of CNT aggregates produced in ReferenceExample 1-2 and Example 1-1, respectively;

FIG. 3 is a graph showing changes in the yield of CNTs depending on Mo:Vratio and calcination temperature in Reference Examples 1-1 and 1-2 andExamples 1-1, 1-2, and 1-3; and

FIG. 4 is a graph showing changes in the yield of CNTs as a function ofreaction time in Example 3 and Reference Example 2.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described in more detail.

The present invention provides an impregnated supported catalystprepared by sequentially adding a multi-carboxylic acid and precursorsof first and second catalytic components to precursors of first andsecond active components to obtain a transparent aqueous metal solution,impregnating an aluminum-based granular support with the transparentaqueous metal solution, followed by drying and calcination, wherein thesupported catalyst has a bulk density of 0.8 to 1.5 g/cm³.

The present invention also provides a carbon nanotube aggregateincluding a four-component catalyst in which catalytic components andactive components are supported on a granular support, and bundle typecarbon nanotubes grown on the catalyst wherein the carbon nanotubeaggregate has an average particle diameter of 100 to 800 μm, a bulkdensity of 80 to 250 kg/m³, and a spherical or potato-like shape.

The bulk density can be defined by Expression 1:

Bulk density=CNT weight (kg)/CNT volume (m³)  (1)

where CNT refers to the carbon nanotube aggregate.

According to the present invention, the use of the four-componentcatalyst with little fine powder allows the carbon nanotubes grown onthe catalyst to have a density distribution and an average particlediameter in specific ranges.

The carbon nanotubes of the carbon nanotube aggregate according to thepresent invention may have a strand diameter of 10 to 50 nm.

The bundle type carbon nanotubes grown on the aluminum-based granularcatalyst have a particle size distribution (Dcnt) of 0.5 to 1.0.

Unless otherwise mentioned, the term “bundle type carbon nanotubes” usedherein refers to a type of carbon nanotubes in which the carbonnanotubes are arranged in parallel or get entangled to form bundles orropes.

The particle size distribution (Dcnt) can be defined by Expression 2:

Dcnt=[Dn90−Dn10]/Dn50  (2)

where Dn90, Dn10, and Dn50 are the number average particle diameters ofthe CNTs after standing in distilled water for 3 hours, as measuredunder 90%, 10%, and 50% in the absorption mode using a particle sizeanalyzer (Microtrac), respectively.

The particle size distribution may be, for example, from 0.55 to 0.95 orfrom 0.55 to 0.9.

The bundle type carbon nanotubes may have an aspect ratio of 0.9 to 1.The bundle type carbon nanotubes may have a diameter of 1 to 50 μm. Theaspect ratio range and the type of the carbon nanotubes can be achievedby a specific process of the four-component catalyst, which is presentedin the present invention. Specifically, the aspect ratio is defined byExpression 3:

Aspect ratio=the shortest diameter passing through the center of CNT/thelongest diameter passing through the center of CNT  (3)

where CNT refers to the carbon nanotube aggregate.

In the catalyst used for the production of the carbon nanotube assembly,catalytic components and active components are supported on a granularsupport. The catalytic components include first and second catalyticcomponents. The active components include first and second activecomponents. The four-component catalyst is prepared by calcination ofthe catalytic components and the active components. The number of moles(x) of the first catalytic component, the number of moles (y) of thesecond catalytic component, the number of moles (p) of the first activecomponent, and the number of moles (q) of the second active componentwith respect to 100 moles of the support may satisfy the followingrelationships:

1≦y≦20;

0.1≦y/[x+y]≦0.5;

1≦p+q≦20; and

0.1≦[p+q]/[x+y]≦0.5.

In a preferred embodiment of the present invention, based on 100 molesof the aluminum-based support, the number of moles (x) of the firstcatalytic component and the number of moles (y) of the second catalyticcomponent satisfy 30≦x+y≦53, and the number of moles (p) of the firstactive component and the number of moles (q) of the second activecomponent satisfy 3≦p+q≦13. More preferably, based on 100 moles of thealuminum-based support, x and y satisfy 30≦x+y≦44 or 35≦x+y≦44, and pand q satisfy 3≦z≦9.5 or 5≦z≦9.5.

In a preferred embodiment of the present invention, the supportedcatalyst is a calcined catalyst in which the catalytic components andthe active components are supported on the granular support. Thegranular support may have an aspect ratio of 1.2 or less, and theaverage aspect ratio (As) of the support before the catalytic componentsand the active components are supported on the support and the averageaspect ratio (A_(CAT)) of the catalyst after the catalytic componentsand the active components are supported on the support may satisfy0.8≦A_(CAT)/As≦1.2.

According to a preferred embodiment of the present invention, thegranular support may have a bulk density of 0.6 to 1.2 g/cm³, and thesupported catalyst prepared by impregnation of the catalytic componentsand the active components into the granular support may have a bulkdensity of 0.8 to 1.5 g/cm³.

The supported catalyst of the present invention has a structure in whichthe catalytic components and the active components are uniformlyimpregnated into and coated on the surface and pores of the support. Dueto this structure, the amount of a fine powder as an aggregate of thecatalytic metals remaining uncoated can be reduced to less than 5% andthe spherical or potato-like shape of the support is maintainedunchanged even after completion of the catalyst production. Thespherical or potato-like shape refers to a three-dimensional shapehaving an aspect ratio of 1.2 or less such as a sphere or ellipse.

The catalyst of the present invention may have a particle diameter (oraverage particle diameter) of 30 to 150 μm, as measured before thecalcination, and each of the support and catalyst may have a sphericalor potato-like shape with a primary particle diameter of 10 to 50 nm.

According to one embodiment, the first catalytic component may beselected from Fe, Ni, and a mixture thereof, the second catalyticcomponent may be Co, the first active component may be Mo, and thesecond active component may be V. Particularly, the addition of vanadium(V) as the second active component leads to a high yield of carbonnanotubes and a significant increase in the density (or bulk density) ofcarbon nanotubes.

The catalytic components used in the present invention may include atleast one metal selected from Fe and Ni as the first catalytic componentand Co as the second catalytic component. For example, the firstcatalytic component may be selected from the group consisting of Fesalts, Fe oxides, Fe compounds, Ni salts, Ni oxides, Ni compounds, andmixtures thereof, and the second catalytic component may be selectedfrom the group consisting of Co salts, Co oxides, Co compounds, andmixtures thereof. As another example, the first catalytic component maybe selected from the group consisting of Fe(NO₃)₂.6H₂O, Fe(NO₃)₂.9H₂O,Fe(NO₃)₃, Fe(OAc)₂, Ni(NO₃)₂.6H₂O, and mixtures thereof, and the secondcatalytic component may be selected from the group consisting ofCo(NO₃)₂.6H₂O, Co₂(CO)₈, [Co₂(CO)₆(t-BuC═CH)], Co(OAc)₂, and mixturesthereof.

The first active component may be Mo, for example, a Mo salt, a Mooxide, or a Mo compound. As another example, the first active componentmay be (NH₄)₆Mo₇O₂₄.4H₂O, Mo(CO)₆, or (NH₄)MoS₄, which may be dissolvedin distilled water before use.

The second active component may be V, for example, a V salt, a V oxide,or a V compound. As another example, the second active component may beNH₄VO₃, which may be dissolved in distilled water before use.

The total amount of the first and second active components may be from0.2 to 4% by weight, based on the weight of the aqueous metal solution.

The first active component (Mo) and the second active component (V) maybe in a weight ratio of 6-0.1:0.1-6, more preferably 5-1:2-4. Thecatalyst essentially includes molybdenum (Mo) and vanadium (V) as theactive components. The ratio of the metal components may be controlledto obtain carbon nanotubes in a high yield, for example, 5000% or more,6000% or more, or 7500% or more.

The four-component catalyst of the present invention has a structure inwhich the surface and pores of the aluminum-based support are coatedwith a monolayer or multilayer of the catalytic components and theactive components, and the amount of a fine powder after ultrasonicationis 10% or less of the amount of the catalyst. Accordingly, the densitydistribution of the carbon nanotubes grown on the catalyst is much moreuniform than that of carbon nanotubes grown on a conventional catalyst.In one embodiment of the present invention, the amount of a fine powderhaving a number average particle diameter not larger than 32 μm, asmeasured after ultrasonic shaking at 40 watts for 1 minute, may be 10%or less, preferably 5% or less, of the amount of the catalyst takinginto consideration the particle diameter (or average particle diameter)range (32-95 μm) of the aluminum-based support.

For reference, the fine powder is defined as an aggregate of thecatalytic materials and the active materials attached to the catalystafter ultrasonication. The fine powder passes through a sieve but isfundamentally different from the catalytically active materialswell-coated on the support in terms of particle size and catalyticactivity. The fine powder is an island-like aggregate attached to thecatalyst and is a cause of low CNT yield. Portions of the catalyticmaterials and the active materials are somewhat weakly attached to thecatalyst and are thus separated from the catalyst during ultrasonicationto form the fine powder.

The supported catalyst of the present invention is preferably preparedby an impregnation method for the following reasons: the supportedcatalyst has a higher inherent bulk density than coprecipitatedcatalysts; unlike coprecipitated catalysts, the supported catalystproduces a small amount of a fine powder with a size of 10 microns orless, which reduces the possibility of occurrence of a fine powder dueto attrition during fluidization; and high mechanical strength of thesupported catalyst effectively stabilizes the operation of a fluidizedbed reactor.

The aluminum-based granular support used in the catalyst of the presentinvention may be made of an aluminum compound selected from the groupconsisting of Al₂O₃, AlO(OH), Al(OH)₃, and mixtures thereof. Thealuminum-based granular support is preferably made of alumina (Al₂O₃).The support may be in the form of a powder instead of a water-solublesalt such as aluminum nitrate. The use of the support in the form of apowder allows the impregnated supported catalyst to have a very highbulk density of 0.5 to 1.5 g/cm³. This is considered a significantdifference between the impregnated catalyst and coprecipitatedcatalysts. The high bulk density of the catalyst enables the operationof a reactor at a high linear velocity in the production of carbonnanotubes and serves to markedly increase the hourly productivity ofcarbon nanotubes.

The aluminum (Al)-based support may further include at least one oxideselected from the group consisting of ZrO₂, MgO, and SiO₂. The aluminum(Al)-based granular support has a spherical or potato-like shape. Thematerial for the aluminum (Al)-based granular support has a structuresuitable to provide a relatively high surface area per unit weight orvolume, such as a porous structure, a molecular sieve structure, or ahoneycomb structure.

According to a preferred embodiment of the present invention, thegranular support may have a primary particle diameter of 10 to 50 nm, aporosity of 0.1 to 1.0 cm³/g, and a specific surface area of 100 to 300m²/g.

As described above, the supported catalyst may be prepared bysequentially adding a multi-carboxylic acid and the precursors of firstand second catalytic components to the precursors of first and secondactive components to obtain a transparent aqueous metal solution,providing the transparent aqueous metal solution to the aluminum-basedsupport, followed by drying under vacuum and calcination.

The transparent aqueous metal solution means an aqueous solution free ofprecipitates. The term “precipitates” means, for example, deep yellowprecipitates such as Fe(MoO)_(3↓) or dark red precipitates such asFe(VO₃)_(3↓) formed as a result of the reaction of Fe³⁺ with 3MoO⁻ or3VO₃ ⁻ at room temperature when an iron (Fe) precursor (such as ironnitrate) as the catalytic component is added to water and a Mo precursor(such as ammonium molybdate) and a V precursor (such as ammoniumvanadate) as the active components are added thereto.

The multi-carboxylic acid used in the present invention is a compoundhaving one or more carboxyl groups. The multi-carboxylic acid is highlysoluble and serves as a complexing agent to suppress the formation ofprecipitates and to facilitate the synthesis of the catalyst. Themulti-carboxylic acid also serves as an activator to promote thesynthesis of carbon nanotubes.

The multi-carboxylic acid may be selected from dicarboxylic acids,tricarboxylic acids, tetracarboxylic acids, and mixtures thereof.Examples of such multi-carboxylic acids include citric acid, oxalicacid, succinic acid, and tartaric acid. The multi-carboxylic acid may beused in an amount of 0.1 to 1.5% by weight, based on the weight of theaqueous metal solution. The ratio of the moles of the multi-carboxylicacid to the sum of the moles of the first and second active componentsis 0.2-2.0:1, more preferably 0.2-1.0:1, most preferably 0.2-0.5:1.Within this range, no precipitation may take place in the aqueous metalsolution and no cracks may be caused during calcination.

The order of addition of the multi-carboxylic acid and the catalyticcomponents may be changed. For example, the multi-carboxylic acid may beadded to a Mo component and/or a V component before the addition of a Fecomponent or a Co component is added. In this case, the formation ofprecipitates is suppressed, and as a result, the area of the supportsurface covered by precipitates is reduced, resulting in an improvementin the activity of the catalyst.

Specifically, the supported catalyst of the present invention may beprepared by: sequentially blending the multi-carboxylic acid componentand an aqueous solution of the precursors of first and second catalyticcomponents with an aqueous solution of the precursors of first andsecond active components to obtain a transparent aqueous metal solution,and mixing the aluminum-based support with the transparent aqueous metalsolution; and drying the mixture under vacuum at 40 to 80° C. andcalcining the dried mixture at 650 to 800° C. In the supported catalystfor carbon nanotube production, the surface and pores of thealuminum-based support are impregnated and coated with the catalyticcomponents and the active components.

The drying under vacuum may be performed by rotary evaporation at atemperature of 40 to 80° C. for 30 minutes to 3 hours.

Thereafter, the calcination may be performed at a temperature of 650 to800° C., preferably 700 to 750° C. The calcination time is not limitedand may be, for example, in the range of 30 minutes to 15 hours. Withinthese ranges, a large amount of the catalyst can be synthesized in ashort time and the catalytic components and the active components can beuniformly dispersed on the surface of the aluminum-based support.

It is preferred to add the multi-carboxylic acid to the aqueous solutionof the precursors of first and second active components before additionof the aqueous solution of the precursors of first and second catalyticcomponents. In the case where the order of addition of themulti-carboxylic acid and the catalytic components is controlled suchthat the multi-carboxylic acid is added to the aqueous solution of theprecursors of active components before addition of the aqueous solutionof the precursors of catalytic components, the formation of precipitatesis suppressed, and as a result, the area of the support surface coveredby precipitates is reduced, resulting in an improvement in the activityof the catalyst. The transparent aqueous metal solution thus obtainedhas a concentration of 0.01 to 0.4 g/ml, specifically 0.01 to 0.3 g/ml,which is efficient in terms of reactivity.

According to one embodiment, the method may further include aging withrotation or stirring at 45 to 80° C. before the drying under vacuum. Theaging may be performed for a maximum of 5 hours, for example, 20 minutesto 5 hours or 1 to 4 hours.

The method may further include preliminarily calcining the vacuum-driedmixture at 250 to 400° C. before the calcination.

A portion of the total amount of the aqueous metal solution may beimpregnated into the aluminum-based support just before the preliminarycalcination and the remainder of the aqueous metal solution may beimpregnated into the aluminum-based support just before the calcination.Specifically, it is preferred in terms of reaction efficiency that amaximum of 50 vol %, for example, 1 to 45 vol % or 5 to 40 vol %, of theaqueous metal solution is impregnated into the aluminum-based supportjust before the preliminary calcination and the remainder of the aqueousmetal solution is impregnated into the aluminum-based support justbefore the calcination.

The carbon nanotube aggregate of the present invention may be producedby a method including: feeding the supported catalyst for carbonnanotube production into a fluidized bed reactor and introducing atleast one carbon source selected from C₁-C₄ saturated or unsaturatedhydrocarbons, and optionally together with a mixed gas of hydrogen andnitrogen, into the reactor at 500 to 900° C.; and decomposing the carbonsource and growing carbon nanotubes on the catalyst surface by chemicalvapor synthesis.

According to the chemical vapor synthesis, the catalyst for carbonnanotube production is charged into the reactor and the carbon source isthen supplied to the reactor at ambient pressure and high temperature toproduce carbon nanotubes. The hydrocarbon is thermally decomposed and isinfiltrated into and saturated in the catalyst particles. Carbon isdeposited from the saturated catalyst particles and grows into carbonnanotubes.

According to one embodiment, carbon nanotubes may be grown for 30minutes to 8 hours after the carbon source is introduced into thecatalyst for carbon nanotube production. According to the presentinvention, the yield of bundles of carbon nanotubes increases and therate of increase in yield decreases moderately with increasing reactiontime, enabling control over yield depending on processing time andachieving a yield of 5000% or more, 6000% or more, 7500% or more,presumably 10,000% or more, without being bound by processing time.

The carbon source may be a C₁-C₄ saturated or unsaturated hydrocarbon.Examples of such hydrocarbons include, but are not limited to, ethylene(C₂H₄), acetylene (C₂H₂), methane (C₂H₄), and propane (C₃H₈). The mixedgas of hydrogen and nitrogen transports the carbon source, preventscarbon nanotubes from burning at high temperature, and assists in thedecomposition of the carbon source.

The use of the supported catalyst according to the present invention forthe synthesis of carbon nanotubes enables the formation of an aggregateof carbon nanotubes grown on the spherical or potato-like catalystwithout changing the shape of the catalyst. As a result, the carbonnanotubes have a high bulk density while maintaining a normaldistribution in particle size. That is, the carbon nanotubes increase insize without substantial change in the catalyst shape. Therefore, theaverage aspect ratio (A_(CAT)) of the catalyst may be 1.2 or less andthe average aspect ratio (A_(CNT)) of the carbon nanotube aggregate mayalso be 1.2 or less.

The following examples are provided to assist in understanding theinvention. However, it will be obvious to those skilled in the art thatthese examples are merely illustrative and various modifications andchanges are possible without departing from the scope and spirit of theinvention. Accordingly, it should be understood that such modificationsand changes are encompassed within the scope of the appended claims.

Example 1 Production of CNTs Depending on the Weight Ratio of Mo:VExample 1-1 Metal Catalyst (Mo:V=3:3)

A. Preparation of Aqueous Metal Solution

A four-component metal catalyst having a combination of Co and Fe ascatalytic components and Mo and V as active components was prepared bythe following procedure. 0.055 g of (NH₄)₆Mo₇O₂₄.H₂O as a Mo precursorand 0.069 g of NH₄VO₃ as a V precursor were dissolved in 20 ml of waterin flask A, and then 0.037 g of citric acid as a multi-carboxylic acid,2.175 g of Co(NO₃)₂.H₂O as a Co precursor, and 0.318 g of Fe(NO₃)₂.H₂Oas a Fe precursor were added thereto to prepare an aqueous metalsolution.

The aqueous metal solution was observed to be clear and free ofprecipitates. Since 7 moles of Mo was present in one mole of(NH₄)₆Mo₇O₂₄, the number of moles of the active components Mo and V andthe multi-carboxylic acid were 0.3127, 0.5889, and 0.1926 moles,respectively, indicating that the molar ratio of the multi-carboxylicacid to the active components was 0.21:1.

The molar ratio of Co:Fe was fixed to 30:8 and the weight ratio of Mo:Vwas adjusted to 3:3.

B. Preparation of Support

2.5 g of Al₂O₃ (D50v=76 microns, pore volume: 0.64 cm³/g, surface area:237 m²/g, Saint Gobain) as an aluminum-based support was placed in flaskB.

C. Preparation of Supported Catalyst Having First Catalyst Layer

One half (10.6 g) of the total amount (21.3 g) of the solution in flaskA was added to flask B. The catalytically active metal precursors weresufficiently supported on the Al₂O₃, followed by aging with stirring ina thermostatic bath at 60° C. for 5 min. The mixture was dried withrotation at 150 rpm under vacuum for 30 min while maintaining thetemperature. The dried mixture was calcined at 350° C. for 1 h toprepare a homogeneous supported catalyst.

When the number of moles of the Al₂O₃ (2.5 g) was assumed to be 100moles, the numbers of moles of Fe, Co, Mo, and V were 8, 30, 3, and 6moles, respectively.

D. Preparation of Supported Catalyst Having Second Metal Catalyst Layer

The catalyst having the first metal catalyst layer was placed in flaskC, and the other half (10.6 g) of the metal solution in flask A wasadded thereto. The catalytically active metal precursors weresufficiently supported on Al₂O₃, followed by aging with stirring in athermostatic bath at 60° C. for 5 min.

The mixture was dried with rotation at 150 rpm under vacuum for 30 minwhile maintaining the temperature. The dried mixture was calcined at725° C. for 3 h to prepare a homogeneous catalyst.

The catalyst was passed through a 32-micron sieve and the passedparticles were weighed to calculate the content of a fine powder, whichwas defined as the aggregate of the particles. The calculated content ofthe fine powder was 0 wt %. After dispersion in water and ultrasonicshaking at 40 watts for 1 min, the proportion of particles having a sizeof 32 μm or less was measured using a particle size analyzer (Microtrac,bluewave). As a result, the amount of the fine powder afterultrasonication corresponded to 0% on the basis of number averageparticle diameter.

E. CNT Synthesis

A test for the synthesis of carbon nanotubes using the catalyst preparedin D was conducted in a fluidized bed reactor on a laboratory scale.Specifically, the catalyst was mounted at the center of a quartz tubehaving an inner diameter of 55 mm and heated to 700° C. under a nitrogenatmosphere. A mixed gas of nitrogen, hydrogen and ethylene gas in thesame volume ratio was allowed to flow at a rate of 180 ml/min for atotal of 1 h while maintaining the same temperature, affording a carbonnanotube aggregate.

Example 1-2 Metal Catalyst (Mo:V=4.5:1.5)

The procedure of Example 1-1 was repeated except that the weight ratioof Mo:V was adjusted to 4.5:1.5.

Example 1-3 Metal Catalyst (Mo:V=4:2)

The procedure of Example 1-1 was repeated except that the weight ratioof Mo:V was adjusted to 4:2.

Reference Example 1 Production of CNTs Using Trimetallic Catalyst(Co—Fe—Mo or V) Reference Example 1-1 Trimetallic Catalyst (Co—Fe—Mo)

The procedure of Example 1-1 was repeated except that the weight ratioof Mo:V was adjusted to 6:0.

Reference Example 1-2 Trimetallic Catalyst (Co—Fe—V)

The procedure of Example 1-1 was repeated except that the weight ratioof Mo:V was adjusted to 0:6.

SEM Images

The CNT aggregates of Example 1-1 and Reference Example 1-1 wereobserved using FE-SEM (HITACHI S-4800, Cold cathode field emission gun,3-stage electromagnetic lens system, SE detector) at an acceleratedvoltage of 5 kV, an emission current of 10 μA, and a working distance of8 mm, and the SEM images are shown in FIGS. 1, 2 a and 2 b,respectively.

FIG. 1 shows a bulk shape of the CNTs produced in Example 1-1. The CNTswere observed to be potato-like or spherical in bulk shape.

The support had an average aspect ratio (As) of 1.2 or less before thecatalytic components and the active components were supported thereon.The average aspect ratio (A_(CAT)) of the catalyst after the catalyticcomponents and the active components were supported thereon and theaverage aspect ratio (As) of the support were shown to satisfy0.8≦A_(CAT)/As≦1.2.

FIG. 2 a shows that the CNT aggregate produced using the trimetalliccatalyst (Co—Fe—Mo) in Reference Example 1-1 consisted of a number ofCNTs, which simply got entangled and were random in shape. In contrast,FIG. 2 b shows that the CNT aggregate produced using the tetrametalliccatalyst (Co—Fe—Mo-V) in Example 1-1 consisted of a number of CNTs,which were grown with high density, had a spherical or potato-like bulkshape, and aggregated regularly to form bundles or ropes.

Bulk Densities of the Catalysts

Each of the catalysts was filled in a measuring cylinder and weighed.

The weight was divided by the volume of the measuring cylinder. As aresult, the catalysts of Example 1-1, Example 1-2, and Example 1-3 wereconfirmed to have bulk densities of 1.0 g/cm³, 1.2 g/cm³, and 1.1 g/cm³,respectively.

Bulk Densities of CNTs

The CNTs were filled in a measuring cylinder and weighed. The weight wasdivided by the volume of the measuring cylinder. As a result, the CNTsof Example 1-1, Example 1-2, and Example 1-3 were confirmed to have bulkdensities of 210 kg/m³, 183 kg/m³, and 170 kg/m³, respectively.

Aspect Ratios

The longest diameter and the shortest diameter passing through thecenter of CNT were measured from the corresponding SEM image. The aspectratio of the CNT was determined by dividing the longest diameter by theshortest diameter. The CNTs of Example 1-1, Example 1-2, and Example 1-3were confirmed to have aspect ratios of 0.90, 0.95, and 1.0,respectively.

Particle Diameter Distributions (Dcnt)

Each of the CNT aggregates was allowed to stand in distilled water for 3h. The number average particle diameter of the CNT aggregate wasmeasured in the absorption mode using a particle size analyzer(Microtrac) and was substituted into Formula 1 to calculate the particlediameter distribution (Dcnt) of the CNTs. The CNTs of Example 1-1,Example 1-2, and Example 1-3 were confirmed to have particle diameterdistributions (Dcnt) of 0.88, 0.92, and 0.95, respectively.

The experimental results are summarized in Table 1.

TABLE 1 Bulk Bulk Particle Weight density of density of diameter x y p qratio of catalyst CNTs Aspect distribution (mole) (mole) (mole) (mole)Mo:V (g/cm³) (kg/m³) ratio (Dcnt) Example 1-1 30 8 3 6 3:3 1.0 210 0.900.88 Example 1-2 30 8 5.5 3.5 4.5:1.5 1.2 183 0.95 0.92 Example 1-3 30 86 3 4:2 1.1 170 1.0 0.95

Reaction Yield Measurement

The contents of the carbon nanotubes obtained at room temperature weremeasured using an electronic scale. The reaction yield was calculated bysubstituting the weight of the catalyst used for CNT synthesis and thetotal weight after the reaction into the following expression:

CNT yield (%)=(the total weight after the reaction (g)−the weight of thecatalyst used (g))/the weight of the catalyst used (g)×100

Referring to FIG. 3, the yield of the CNTs produced using the metalcatalyst consisting of Co, Fe, and Mo in Reference Example 1-1 was about3500%, and the yield of the CNTs produced using the metal catalystconsisting of Co, Fe, and V in Reference Example 1-2 was lower than5000%. In contrast, the yield of the CNTs produced using the metalcatalyst including Mo and V in a ratio of 4.5:1.5 in Example 1-2 was6000% or more, the yield of the CNTs produced using the metal catalystincluding Mo and V in a ratio of 4:2 in Example 1-3 was 6500% or more,and the yield of the CNTs produced using the metal catalyst including Moand V in a ratio of 3:3 in Example 1-1 was close to 5000%.

Example 2 CNT Yield Depending on Calcination Temperature Example 2-1Mo:V=3:3

The procedure of Example 1-1 was repeated except that the metal catalystwas calcined at 710° C.

Example 2-2 Mo:V=4.5:1.5

The procedure of Example 1-2 was repeated except that the metal catalystwas calcined at 710° C.

Example 2-3 Mo:V=4:2

The procedure of Example 1-3 was repeated except that the metal catalystwas calcined at 710° C.

Referring to FIG. 3, the yield of the CNTs produced in Example 2-2 wascomparable to the yield of the CNTs produced in Example 1-2. The CNTswere produced using the catalysts including Mo and V in the same weightratio (4.5:1.5), except that the calcination temperatures were different(710° C. in Example 2-2 and 725° C. in Example 1-2). In contrast, theyield (≧7500%) of the CNTs produced in Example 2-3 was higher by ≧+1000%than that of the CNTs produced in Example 1-3. The CNTs were producedusing the catalysts including Mo and V in the same weight ratio (4:2),except that the calcination temperatures were different (710° C. inExample 2-3 and 725° C. in Example 1-3). The yield (˜6900%) of the CNTsproduced in Example 2-1 was higher by ≧+2000% than that of the CNTsproduced in Example 1-1. The CNTs were produced using the catalystsincluding Mo and V in the same weight ratio (3:3), except that thecalcination temperatures were different (710° C. in Example 2-1 and 725°C. in Example 1-1).

Example 3 CNT Yield Depending on Reaction Time

CNTs were synthesized in the same manner as in Example 1-1, except thatthe reaction time was changed. Changes in the CNT yield as a function ofreaction time from 1 to 8 h were observed.

Reference Example 2 CNT Yield Depending on Reaction Time

CNTs were synthesized in the same manner as in Reference Example 1-1,except that the reaction time was changed. Changes in the CNT yield as afunction of reaction time from 1 to 8 h were observed.

Referring to FIG. 4, when the reaction time was 1 h, the use of thefour-component catalyst (cobalt (Co)-iron (Fe)-molybdenum (Mo)-vanadium(V)) led to an at least 30% increase in CNT yield compared to the use ofthe three-component catalyst (Co—Fe—Mo). In addition, when thethree-component catalyst (Co—Fe—Mo) was used, the rate of increase inyield steeply decreased with increasing reaction time. It could be thusexpected that the yield would not be increased any more when thereaction time reaches 8 h. In contrast, the use of the four-componentcatalyst (cobalt (Co)-iron (Fe)-molybdenum (Mo)-vanadium (V)) led to ahigh yield and moderately decreased the rate of increase in yield withincreasing reaction time, allowing the reaction to proceed for a longertime, for example, 8 h or more or 10 h or more, and enabling theproduction of CNT bundles in higher yield.

The present invention can overcome the disadvantage of low CNT yieldencountered in the use of conventional impregnated catalysts for carbonnanotube production. The supported catalyst of the present invention hascontrolled activity and can produce a controlled amount of a finepowder, enabling the synthesis of bundle type carbon nanotubes in highyield. Therefore, the supported catalyst of the present invention canfind application in various fields, such as energy materials, functionalcomposites, pharmaceuticals, batteries, and semiconductors, where carbonnanotubes are used.

1. An impregnated supported catalyst prepared by sequentially adding amulti-carboxylic acid and precursors of first and second catalyticcomponents to precursors of first and second active components to obtaina transparent aqueous metal solution, impregnating an aluminum-basedgranular support with the transparent aqueous metal solution, followedby drying and calcination, wherein the supported catalyst has a bulkdensity of 0.8 to 1.5 g/cm³.
 2. The impregnated supported catalystaccording to claim 1, wherein the catalyst comprises first and secondcatalytic components and first and second active components, and thenumber of moles (x) of the first catalytic component, the number ofmoles (y) of the second catalytic component, the number of moles (p) ofthe first active component, and the number of moles (q) of the secondactive component with respect to 100 moles of the support satisfy thefollowing relationships:10≦x≦40;1≦y≦20;0.1≦y/[x+y]≦0.5;1≦p+q≦20; and0.1≦[p+q]/[x+y]≦0.5.
 3. The impregnated supported catalyst according toclaim 1, wherein the granular support has a bulk density of 0.6 to 1.2g/cm³, and the catalyst in which the catalytic components and the activecomponents are supported has a bulk density of 0.8 to 1.5 g/cm³.
 4. Theimpregnated supported catalyst according to claim 1, wherein thegranular support has an aspect ratio of 1.2 or less, and the averageaspect ratio (As) of the support before the catalytic components and theactive components are supported on the support and the average aspectratio (A_(CAT)) of the catalyst after the catalytic components and theactive components are supported on the support satisfy0.8≦A_(CAT)/As≦1.2.
 5. The impregnated supported catalyst according toclaim 1, wherein the multi-carboxylic acid is used in an amount of 0.2to 2.0 moles, assuming that the sum of the moles (p+q) of the first andsecond active components equals to
 1. 6. The impregnated supportedcatalyst according to claim 1, wherein the multi-carboxylic acid isselected from dicarboxylic acids, tricarboxylic acids, tetracarboxylicacids, and mixtures thereof.
 7. The impregnated supported catalystaccording to claim 1, wherein the calcination is performed at 650 to800° C.
 8. The impregnated supported catalyst according to claim 1,wherein the first catalytic component is cobalt (Co), the secondcatalytic component is selected from iron (Fe), nickel (Ni), and amixture thereof, the first active component is molybdenum (Mo), and thesecond active component is vanadium (V).
 9. The impregnated supportedcatalyst according to claim 1, wherein the first and second activecomponents are in a weight ratio of 6-0.1:0.1-6.
 10. The impregnatedsupported catalyst according to claim 1, wherein the catalyst has astructure in which the surface and pores of the aluminum-based supportare coated with a monolayer or multilayer of the catalytic componentsand the active components, and the amount of a fine powder having anumber average particle diameter not larger than 32 μm, as measuredafter ultrasonic shaking at 40 watts for 1 minute, is 10% or less of theamount of the catalyst.
 11. The impregnated supported catalyst accordingto claim 6, wherein the transparent aqueous metal solution has aconcentration of 0.01 to 0.4 g/ml.
 12. A carbon nanotube aggregatecomprising the impregnated supported catalyst according to claim 1 andbundle type carbon nanotubes grown on the catalyst wherein the carbonnanotube aggregate has an average particle diameter of 100 to 800 μm, abulk density of 80 to 250 kg/m³, and a spherical or potato-like shape.13. The carbon nanotube aggregate according to claim 12, wherein thecarbon nanotubes have an aspect ratio of 0.9 to 1 and a strand diameterof 10 to 50 nm.
 14. The carbon nanotube aggregate according to claim 12,wherein the catalyst has an average aspect ratio (A_(CAT)) of 1.2 orless and the carbon nanotube aggregate has an average aspect ratio(A_(CNT)) of 1.2 or less.
 15. The carbon nanotube aggregate according toclaim 12, wherein the bundle type carbon nanotubes have a particle sizedistribution (Dcnt) of 0.5 to 1.0.
 16. A method for producing a carbonnanotube aggregate, comprising: 1) sequentially blending amulti-carboxylic acid component and an aqueous solution of precursors offirst and second catalytic components with an aqueous solution ofprecursors of first and second active components to obtain a transparentaqueous metal solution, and mixing an aluminum-based granular supportwith the transparent aqueous metal solution; 2) drying the mixture undervacuum at 40 to 80° C. and calcining the dried mixture at 650 to 800° C.to obtain a catalyst for carbon nanotube production in which the surfaceand pores of the aluminum-based support are impregnated and coated withthe catalytic components and the active components; 3) feeding thecatalyst for carbon nanotube production into a fluidized bed reactor andintroducing at least one carbon source selected from C₁-C₄ saturated orunsaturated hydrocarbons, and optionally together with a mixed gashydrogen and nitrogen, into the reactor at 500 to 900° C.; and 4)decomposing the carbon source and growing carbon nanotubes on thecatalyst surface by chemical vapor synthesis.
 17. The method accordingto claim 16, further comprising aging at 45 to 80° C. before the dryingunder vacuum in step 2).
 18. The method according to claim 16, furthercomprising preliminarily calcining at 250 to 400° C. before thecalcination in step 2).
 19. The method according to claim 18, furthercomprising impregnating a portion of the total amount of the aqueousmetal solution into the aluminum-based granular support just before thepreliminary calcination and impregnating the remainder of the aqueousmetal solution into the aluminum-based granular support just before thecalcination.